The present invention relates generally to methods for transmitting digital information between a main unit and a number of subunits of an electrical device, and preferably to a method for transmitting digital information between a main unit and subunits of a modular programmable controller. The method of the present invention transmits digital information via a serial bus system having at least one data line for transmitting the information, one timing line for specifying a shared system clock, and at least one control line for transmitting control signals. In the method of the present invention, at least the data line may run through the subunits so that the bus system is operable as a shift register having a total register length equal to the sum of individual register lengths of the subunits. The present invention also relates generally to electrical devices for implementing the method.
German 36 03 751 A1 discloses a method for transmitting digital information between a main unit and a number of subunits. The method applied therein for transmitting information via a circulating register is not yet quite optimal, since in order to transmit the information, the entire register must always be cleared, and the register lengths of the individual modules or peripheral units must be definitively preset. Thus, when information is supposed to be transmitted, for example, to only one of the peripheral units, a great deal of blank information must be transmitted along with it. This reduces the theoretically attainable speed of information transmission.
The present invention is directed to the problem of developing a method for transmission of digital information over a serial bus system, which is designed as a shift register, yet which method operates as flexibly, efficiently, and almost as user-friendly as a parallel bus system.